In hybrid cars and electric vehicles which are required to have large power and large capacity, it is necessary to form a battery pack from an assembly of several tens of or hundred or more unit batteries. For increasing the battery mounting density, the development of a rectangular secondary battery has been made.
For example, the non-aqueous electrolyte secondary battery described in patent document 1 has an electrode group including positive and negative electrode plates each having a current collector foil, and a separator for insulating them, wherein the positive and negative electrode plates and the separator are spirally wound together into a flattened shape. The electrode group has formed at the both ends thereof respective exposed portions having no active material mixture applied onto the positive and negative current collector foils so that the respective metallic foils are exposed. The metallic foil exposed portions are positioned opposite to each other in the electrode group. The external shape of the flattened electrode group has at both ends a pair of curved surface portions around which the positive and negative electrode plates are wound, and top and bottom plane portions which connect the curved surface portions, and the electrode group is inserted into a battery casing so that one of the curved surface portions is disposed on the bottom side of the battery casing.